I. Field of the Invention
The present invention relates to electronics circuits. More particularly, the present invention relates to a novel and improved phase splitter for generating multiple output signals of equal amplitude but different phases using active devices.
II. Description of the Related Art
Phase splitters are circuits which generate multiple output signals of equal amplitude but different phases. Phase splitters are widely used in the electronics industry for a variety of applications. In particular, phase splitters are commonly employed in communication applications. Typical applications include single sideband modulators, image-reject mixers, and IQ modulators and demodulators such as those used in quadrature phase shift keying (QPSK) or offset quadrature phase shift keying (OQPSK) modulation. These applications require a 90.degree. phase splitter wherein two output signals of equal amplitude but delayed by a quarter period relative to each other (or 90.degree. phase difference) are required. For phase splitters, it is the difference in the phases of the output signals, or the phase difference, which is of importance. The absolute phases of the output signals relative to the input signal is usually not important. For an ideal 90.degree. phase splitter, the amplitude response and the phase difference response are flat across the all frequencies from DC to infinity (.infin.) Hz.
Phase splitters can be implemented as a combination of all-pass networks. An all-pass network has a constant or flat amplitude response but the phase response varies over frequency. Two or more all-pass networks can be connected together to a common input, with each network having a different phase response. The outputs of the networks are signals having equal amplitude but different phases. Typically, the networks are selected such that the difference in the phases of the output signals equals a desired value at a specified frequency or frequency range.
A simple implementation of a passive phase splitter can be designed using single-pole RC all-pass networks as shown in FIG. 1A. Within passive phase splitter 2, amplifier 4 provides the inverting gain (A.sub.v =-1) necessary for the operation of the feedback circuit. Resistor R.sub.a 6 and capacitor C.sub.a 10 provide a first phase shift of the input signal V.sub.in (s) and results in the output signal V.sub.a (s) at the node between resistor 6 and capacitor 10. Throughout the specification, the signals and transfer functions are described as functions of s where s is a complex frequency (s=j.omega.). Similarly, resistor R.sub.b 8 and capacitor C.sub.b 12 provide a second phase shift of the input signal V.sub.in (s) and results in the output signal V.sub.b (s). The transfer functions of the output signals V.sub.a (s)/V.sub.in (s) and V.sub.b (s)/V.sub.in (s) have the same gain, thereby resulting in signals V.sub.a (s) and V.sub.b (s) having equal amplitude. However, the phases of V.sub.a (s) and V.sub.b (s) are different and the phase difference can be plotted versus frequency as shown in FIG. 7. The plot shows that at the center frequency .omega..sub.o =2.pi..multidot.130.8 MHz, the phase difference between V.sub.a (s) and V.sub.b (s) is 90.degree..
Typically, the output signals V.sub.a (s) and V.sub.b (s) need to drive another circuit, which is also referred to as a load, in the system. If the impedance of the load is a resistance of finite value, the responses of passive phase splitter 2 will be altered. Passive phase splitter 2 needs to be modified to maintain an accurate balance of the amplitude and phase difference between the two output signals. One possible modification is the addition of resistors in series with C.sub.a and C.sub.b (not shown in FIG. 1A). Alternately, the output signals can be buffered before driving the load.
In the ideal form, without taking into account circuit parasitic effect which are inevitable in any circuit, the transfer function of passive phase splitter 2 results in the desired amplitude and phase difference responses. However, in practice, this circuit has several drawbacks. First, passive phase splitter 2 exhibits power loss due to the presence of resistors 6 and 8 in the circuit. The power loss is usually compensated by adding extra gain stages which increase complexity and power consumption of the system. Second, passive phase splitter 2 exhibits degradation in the noise figure due to the thermal noise in resistors 6 and 8. Third, the transfer functions of passive phase splitter 2 is sensitive to the impedance of the load. Sensitivity to the load impedance can result in degradation in the balance of the amplitude and phase difference. Furthermore, sensitivity to the variations in the load impedance from circuit to circuit can render it difficult to design passive phase splitter 2 for the worse case. And fourth, the phase difference between the outputs of passive phase splitter 2 at a particular frequency depends on the absolute values of resistors 6 and 8. The dependence of the absolute values on process and temperature variations causes variations in the phase difference. The present invention addresses these problems by the use of an active phase splitter.